150 optimax blew up at idle after sitting awhile

[wayneo oz]

Hi Guys.
Well today I decided to start my 09 150hp Optimax after it has been sitting for 6 or so months.
I changed fuel, filters and added treatment.
I backed the boat into my small dam and lowered the tilt to put the engine into correct depth then hit the starter.
After the usual couple of cycles to built air pressure the engine fired straight up then almost immediately began knocking then stopped dead..
I tried to restart it but couldn't.
Then I manually wound the engine over by turning the flywheel and it seemed ok..Again I hit the starter and it fired but stoped dead again.
I then noticed fresh oil running out of the exhaust so alarm bells went off..I pulled all the plugs out and found no 6 cyl had no compression..
So with a heavy heart I began tearing the engine down from the intake side and once I removed the reed plate I found some lovely big chunks of alloy and steel..I am guessing them to be a gudgeon pin and bits of piston.
So my questions to you guys is,
What do you thing would cause such a failure to occur ? It cant be lack of oil because the thing is dripping wet inside and in the bad cyl.
How do I split the crankcase? Do I need to remove the powerhead or can I split the case insitu? I have taken all the bolts visible out but it seems to be still attached strongly..

Thanks for reading and in advance for your help.

 

[racerone]

Power head has to be removed to split the case.-----Your best bet is a new motor.----This will be expensive !

 

[wayneo oz]

Power head has to be removed to split the case.-----Your best bet is a new motor.----This will be expensive !

Thanks for the reply Mate.
I pulled the port side head off today and found a piston with a broken skirt and conpin..
The head is fine as is the bore and the conrod looks unbent although its knocked about from hitting the cyl base.
I am very concerned that this is my second opti with an almost identical blow up result and a piston I kept from my previous 175 opti is almost identical as far as damage goes but that engine failed terribly at speed..Also there is big piston clearance in the bore and the broken piston has blowby marks that will need to be investigated and reasons found because the bores still have very clear hone marks and there is no lip as would occur with wear and the engine has only 350 hrs on it so I'm tipping bad piston bore match from the factory.
I hope I can post some photos for you.
It looks like a rebuild kit , a conrod, a reed set and some serious cleanout is in order..